The present invention relates generally to aqueous compositions which can be coated on active cores (e.g., pharmaceutical preparations, herbicides, chemical reactants, etc.) to provide for the controlled release of an active agent over a period of time. These aqueous coating compositions contain non-toxic materials--a pre-crosslinked polyorganosiloxane, colloidal silica and a water-dispersible organic polymer--which can be coated or sprayed onto active substrates, including water-soluble substrates and dried to form an elastomeric film. Upon exposure to an aqueous environment, the coated substrates of the present invention respond in a manner which causes the active agent to pass into the aqueous surroundings at a substantially constant rate.
Conventional tablets dissolve at a non-uniform rate characterized by an initial rapid introduction of the active ingredient into its surroundings, followed by a relatively low rate of release. Efforts to provide for a more uniform administration of an active agent has prompted the use of sustained-release compositions which employ granules of active ingredients coated with differing thicknesses of cellulose acetate/polyethylene oxide solutions. Controlled release is achieved by collecting granules of varying coating thicknesses into a single capsule.
Another approach of particular interest to the present invention is illustrated in U.S. Pat. No. 4,268,496. The foregoing patent discloses pharmaceutical compositions coated with two-component, aqueous coating compositions comprising water-soluble cellulose derivatives and curable organopolysiloxane or organohydrogenpolysiloxane polymers. The coating compositions disclosed in this patent do not use a reinforcing colloidal silica filler and the patent teaches that organometallic or platinum catalysts can be employed, if necessary, to cure the siloxane polymer.
U.S. Pat. No. 3,935,326 discloses water-based synthetic resins for spraying on tablet-cores. The patent describes both the use of water-soluble substances, including polyethylene glycols, and the possibility of employing highly dispersed silicic acid fillers in the spraying compositions. The resin components disclosed are not silicones, but comprise organic resins which are formed from free radical emulsion polymerizations, are not crosslinked, and require the addition of plasticizers.
European Patent Application 0 171 457 discloses tablet coating compositions comprising a water-dispersible material (e.g., polyethylene glycol) and a semipermeable membrane. The published application indicates that polydialkylsiloxanes can be used to form such membranes, but teaches that the membrane is formed by dispersing the membrane precursor in an organic solvent.